1. Field of the Invention
The present invention relates in general to delivery apparatus for pressurized medical liquids and in particular to an apparatus for pressurizing a medical liquid to a delivery pressure.
2. Description of the Prior Art
In delivery apparatuses for pressurized medical liquids it is known to maintain a medical liquid in a reservoir at a delivery pressure by means of pressurized gas. The gas is inlet to the reservoir from a pressurized gas source. In the reservoir the gas exerts its pressure on the surface of the medical liquid and the pressurized liquid is delivered through a controllable outlet. Such an apparatus is used for example in a vaporizer for liquid anesthetic wherein a delivery apparatus delivers the liquid via an injector coupled to the outlet and into a flow of breathing gas. In this kind of use it is important that the delivery apparatus is capable of delivering a certain volume of liquid with a high degree of repeatability.
The basic idea for a delivery apparatus of this kind is that the delivered volume of liquid shall be dependent only on the differential pressure over the controllable outlet and on the time during which the outlet is open. In practice, however, a part of the pressurizing gas dissolves in the liquid, which in its turn affects the delivered volume due to the fact that bubbles of gas appear in the injector and in its inlet ducts as well as in or in the vicinity of possible filters.
Simplified experiments with this type of vaporizer have shown that the delivered volume of liquid deviates with an increase by about 10% when pressurizing gas has been dissolved in the liquid until saturation. More specifically, the medical liquid used in the experiments was anesthetic agent Isofluoran and the pressurizing gas was air. The reservoir was maintained at an overpressure of 1 bar and the pulse time of the injector was set to 2.5 ms (milliseconds). In the experiment the time to saturation of the medical liquid was about 72 hours. It should be noted that these figures are approximate and serve to give a general understanding of the diffusion process in this context.
In clinical practice, vaporizers are often used in lengthy surgical operations during which pressurizing gas thus dissolve in the medical liquid and thereby affects the repeatability and accuracy of the delivered dose of the medical liquid into the breathing gas. In practice an increase of 10% of the anesthetic liquid delivered to the patient would not have a severe effect on the majority of patients. However, it is always an aim for the anesthesiologist to maintain anesthesia with as low dose as possible and to have an accurate control of the process, which is more difficult if the delivered dose increases over time without changing the settings of the apparatus.
Another problem is that bubbles that are created through a pressure drop in the injector can get jammed for some types of injectors.
There is therefore a need for improvement of the repeatability and accuracy of the delivered dose in such a delivery apparatus.
EP1082973 discloses an unaesthetic vaporizer for dosing a liquid anesthetic by means of a liquid pump. This piece of prior art seeks to solve the above mentioned problem of dosing accuracy by pressurizing the anesthetic agent with a liquid pump and a system of regulating means. In addition the apparatus is provided with a return tube in order to make it easier to control pressure downstream from the liquid pump. However, this solution is complicated and expensive due to high demands on the materials used and the limited life span of the used components. Moreover, this prior art does not overcome the problem of diffusion of gas into the anesthetic liquid since the liquid reservoir is connected to an air tube for admitting inlet of air at atmospheric pressure in order to compensate for a negative pressure drop arising when liquid is pumped out of the reservoir.
EP 1 300 172 discloses a delivery apparatus for pressurized liquid anesthetics. In this prior art document a primary reservoir is devised with primary pressurizing means in order to pressurize the liquid to a delivery pressure basically as described in the background section above. The primary reservoir in its turn is connected to a secondary reservoir so as to deliver pressurized liquid to the secondary reservoir. The secondary reservoir is provided with a secondary pressurizing means devised to supply compensating pressure to the liquid in the secondary reservoir in order to maintain it at the delivery pressure when pressure is reduced in the primary reservoir. The purpose is to be able to temporarily allow interruption of the pressurizing activity in the primary reservoir without interruption of delivery of the pressurized liquid from the secondary reservoir. Thereby the primary reservoir can be refilled at atmospheric pressure during operation of the delivery apparatus.
In a first embodiment the secondary pressurizing means involves the arrangement of a movable section in the shape of a membrane that delimits an inner variable volume liquid containing space and that is used to put pressure on the liquid surface by means of a mechanical or pressurized gas biasing force. In fact the secondary reservoir is designed with a function very similar to that of a piston pump, where the movable section corresponds to the piston and there being provided an inlet valve from the primary reservoir and an outlet valve to the injector. Although not explicitly explained in the disclosure of EP 1 300 172, the movable section must be tightly sealed against the inner walls of the secondary reservoir in order to operate properly and maintain the delivery pressure as intended. For the further purpose of achieving a safety seal that prevents evacuation of content in the secondary reservoir, the movable section is provided with a sealing head that is pressed against an outlet when the movable section is moved to reduce the volume of the space to a minimum.
In a second embodiment the secondary pressurizing means is based on a pressurized gas biasing force similar to that of the first pressurizing means for pressurizing the primary reservoir, namely by introducing pressurized gas through a gas port into the space above liquid in the reservoirs. As stated in the description, this embodiment may optionally also be provided with a movable section, in this instance in the shape of a float provided with a sealing surface, for the purpose of sealing the gas port against escape of liquid in case the space of the reservoir is filled with liquid to a maximum level. This may for example occur if the differential pressure between the primary and the secondary reservoirs is accidentally unbalanced with a higher pressure in the primary reservoir than in the secondary reservoir.
Although the float in the secondary reservoir would prevent its liquid content from becoming saturated with pressurizing gas, there is nothing to prevent saturation in the main, primary reservoir. This piece of prior art does not recognize the problem nor provide a solution with regard to repeatability and accuracy of the delivered dose due to diffusion of pressurizing gas in the medical liquid and there is no teaching in EP 1 300 172 that would give guidance to solve that problem.
The article “The Oxford vaporizer No. 2” by S L Cowan et al, The Lancet, vol. 238, issue 6151, 19 Jul. 1941, pages 64-66 discloses a vaporizer for a medical liquid with a small tank provided with a float for the purpose of closing an orifice that leads to a pump in case the level of liquid in the tank falls to a certain level. The purpose is to prevent entry of air into the pump. There is no mentioning of the problem with diffusion of gas into the medical liquid.
The article “A new respirator” by Claus Bang, The Lancet, vol. 261, issue 6763, 11 Apr. 1953, pages 723-726 discloses a respirator apparatus with an automatic valve for controlling the inlet of pressurized breathing gas into the lungs of a patient. The valve is controlled dependent on the pressure of the lungs by means of an arrangement of two connected and liquid filled tubes each being provided with a float. When the pressure in the lungs reaches lower and higher end values, respectively, one of the floats rises to a certain level and connects two electrodes to close an electric circuit that actuates the automatic valve to open or close. The use of floats in this piece of prior art is totally different from that of the present invention.
WO 89/03483 discloses a hydro-pneumatic accumulator of the float type, preferably for use to store energy in connection with a hydraulic system. Hydraulic or hydro-pneumatic accumulators are used in hydraulic systems to receive and deliver large quantities of working fluid during a short time. The accumulator consists of a vertical container, and a free swimming float which serves to separate the accumulator's content and gas and fluid, and to work together with a seal ring in the end cover as a valve which prevents the gas from escaping from the accumulator if it is completely emptied for fluid. This prior art is mainly directed to the problem of providing a float that has a low specific weight and at the same time is strong enough to withstand the high pressures that appear in hydraulic systems. Although this prior art documents deals with problems that are partly similar to those of the present invention, it is not relevant for the skilled person working with anesthesia delivery apparatuses. Those of ordinary skill in the art would not seek solutions from the technical area of high pressure hydraulic systems and therefore this piece of prior art is not relevant as prior art for the present invention.
WO 99/13228 discloses another hydro-pneumatic accumulator for storing pressure in a large volume container for hydraulic oil. The accumulator is provided with a device that separates the fluid area from the gas area. The separating device contains submerging floating bodies arranged on the upper surface of the fluid volume and are situated in the fluid level itself. In addition, the separating device contains at least one protecting body which interacts with the floating bodies and is configured as a shield for further reduction of the contact surface between gas and fluid. This piece of prior art is directed to the problems met in connection with large volume containers and presents a floating device with a separation element mounted on floating bodies. An important issue for this prior art is that the separating device should be possible to introduce through an inspection opening, usually a manhole. As the previously mentioned prior art, this prior art is directed to high pressure and large scale hydraulic system, and for the same reasons it is not relevant as prior art for the present invention.
Thus, none of the prior art addresses the problem of repeatability and accuracy of delivered dose in a medical delivery apparatus with regard to diffusion of pressurizing gas into a medical liquid.